1. Field of the Invention
This invention relates to a control system for an internal combustion engine provided with an exhaust recirculation (EGR) system for recirculating part of exhaust gases to an intake system, and more particularly to a control system of the type that performs control of fuel injection by an injector, etc.
2. Description of the Related Art
Conventionally, a control system of this kind has been proposed in Japanese Laid-Open Patent Publication (Kokai) No. H08-61112. In the control system, the amount Ga of intake air drawn into cylinders is detected by an air flow sensor. Further, the total amount Gf of gases to be supplied to the cylinders is mapped in advance using the stepped-on amount of an accelerator pedal and the rotational speed of the engine as parameters, and the total amount Gf is calculated by searching the map according to the parameters detected during operation of the engine. Furthermore, the difference (Gf−Ga) between the total amount Gf of gases and the intake air amount Ga is calculated as the amount Ge of EGR gases to be supplied to the cylinders.
On the other hand, an exhaust manifold has an oxygen concentration sensor inserted therein for detecting the concentration of oxygen in exhaust gases.
The amount of air in the EGR gases is calculated based on the detected oxygen concentration in the exhaust gases and the amount Ge of EGR gases, assuming that the oxygen concentration in the exhaust gases detected by the oxygen concentration sensor is equal to the concentration of oxygen in the EGR gases. Then, the sum of the amount of air in the EGR gases and the intake air amount Ga is calculated as the total amount of air to be supplied to the cylinder, and a fuel injection amount is calculated based on the total air amount and a target air-fuel ratio. The EGR operation is carried out under the above-described control, and even when excessive air is contained in the EGR gases, the fuel injection amount is controlled according to the amount of air actually supplied to the cylinders, whereby the air-fuel ratio is controlled to the target air-fuel ratio with accuracy.
As described above, in the above conventional control system, the concentration of oxygen in exhaust gases is regarded as equal to the concentration of oxygen in EGR gases supplied to the cylinders, and the concentration of oxygen in exhaust gases detected by the oxygen concentration sensor is sequentially used for estimation of the amount of air in the EGR gases, estimation of the total amount of air supplied to the cylinders, and calculation of the fuel injection amount. However, there is a limit to the range of the air-fuel ratio detectable by the oxygen concentration sensor. Therefore, since the diesel engine, for example, has a wide range of the air-fuel ratio to be controlled, the operating region of the engine to which the oxygen concentration sensor can be applied is very narrow. Furthermore, the exhaust manifold having the oxygen concentration sensor inserted therein is distant from the cylinders, which produces a time lag before the EGR gases from the exhaust manifold are actually supplied to the cylinders, and the EGR system itself suffers from delay in operation. Therefore, the actual concentration of oxygen in the EGR gases supplied to the cylinders is not necessarily equal to the concentration of oxygen in exhaust gases. Such deviation in the oxygen concentration increases particularly when the amount of the EGR gases is largely changed, e.g. in a transient state of the engine. Further, in the diesel engine, the EGR gas amount is set to a very large value compared with the gasoline engine, and hence the amount of change in the EGR gas amount in the transient state is also large, which causes an increased deviation in the oxygen concentration. This makes it impossible for the control system to properly estimate the amount of air in the EGR gases and the total amount of air supplied to the cylinders, which causes the fuel injection amount to deviate from an appropriate value, thereby making it impossible to accurately control the air-fuel ratio to the target air-fuel ratio.
The Publication of Japanese Patent No. 2600492 discloses an exhaust emission control system which includes a lean NOx catalyst for absorbing NOx from exhaust gases during a lean burn operation in which the air-fuel ratio of a mixture burned in an internal combustion engine is set to a lean air-fuel ratio leaner than the stoichiometric ratio. In this system, the amount of NOx absorbed in the lean NOx catalyst increases as the lean burn operation is continuously performed. Therefore, the air-fuel ratio is made richer than the stoichiometric ratio before the amount of NOx absorbed in the lean NOx catalyst exceeds the allowable limit, whereby the NOx absorbed in the lean NOx catalyst is reduced. Hereinafter, the above enrichment of the air-fuel ratio is referred to as “the Nox-reducing enrichment”.
In the above-described exhaust emission control system disclosed in the Publication of Japanese Patent No. 2600492, when the Nox-reducing enrichment is performed, first, the air-fuel ratio is stepwise switched from the lean air-fuel ratio to a rich air-fuel ratio, and when reduction of NOx is completed, the air-fuel ratio is stepwise returned from the rich air-fuel ratio to the lean air-fuel ratio. Therefore, the system suffers from the problem that it causes engine output torque variation during switching of the air-fuel ratio.
Further, the Publication of Japanese Patent No. 3116876 discloses an internal combustion engine which is configured such that to reduce the amount of NOx in exhaust gases, a relatively large amount of exhaust gases is recirculated to an intake system to thereby lower combustion temperature in the combustion chamber. In this engine, the exhaust gas recirculation rate is set to a higher value than usual, and a relatively large amount of exhaust gases is supplied to the combustion chamber. As a result, the temperature of fuel and gases therearound in the combustion chamber is suppressed to a temperature lower than a temperature at which soot is generated, to thereby prevent generation of soot and reduce the amount of NOx emission.
The low-temperature combustion disclosed in the Publication of Japanese Patent No. 3116876 is possible only in a particular state where load on the engine is low, and hence it is necessary to change the control mode of the engine, particularly the control mode of fuel injection by injectors, along with an increase in load on the engine. However, the Publication of Japanese Patent No. 3116876 does not disclose a method of controlling fuel injection during an increase in the load on the engine, or during transition from a high-load operating condition of the engine to a low-load operating condition thereof under which the low-temperature combustion is possible.